Final answer:
One of Earth's warmest climates occurred 251 million years ago during the Permian period, with conditions conducive to tropical wet forests. The Medieval Warm Period, around 900–1300 AD, also reflected a warmer climate than usual. Detailed climate records have existed for the past 65 million years, marking significant temperature variations throughout Earth's history.
Step-by-step explanation:
One of the warmest climates in Earth's history occurred during the Permian period, which ended around 251 million years ago. This period was one of Earth's three warmest periods and coincided with a significant extinction event, with an estimated 70% of terrestrial plant and animal species and 84% of marine species becoming extinct. It was a time when the planet had average temperatures of around 20°C to 30°C in tropical wet forests, with annual rainfall of 300–400 cm (118–157 in).
Another notable period sometimes referred to when discussing past warm climates is the Medieval Climate Anomaly, also known as the Medieval Warm Period, which occurred between 900 and 1300 AD. During this time, average temperatures were only slightly above the norm, by about 0.10°C to 0.20°C, but this was enough to cause significant changes like freeing seas from ice and allowing the Vikings to colonize Greenland.
When reviewing Earth’s overall climate record, scientists have more detailed information for the past 65 million years of Earth's history. Notably, the climate changes have varied greatly within this timeframe, including the succession of great ice ages and inter-glacial periods such as the one we are currently in, which began about 14,000 years ago after the last ice age ended.
It is important to mention that while the medieval climate anomaly was significant, it does not compare to the extreme warmth of the Permian period or even the steady, gradual temperature rise observed over the 4,000 years leading up to the Industrial Revolution, which contrasts with the rapid temperature increases we have experienced in the past 100 years.